Vascular disorders and defects such as aneurysms and other arteriovenous malformations often occur near the junction of large arteries, for instance at the base of the brain in the Circle of Willis. As aneurysms develop they typically form as a saccular aneurysm protruding from a wall of a vessel and have a neck and a dome portion. Alternatively, aneurysms can form as fusiform malformations that balloon a cross-section of the affected vessel.
As an aneurysm develops, the arterial internal elastic lamina disappears at the base of the neck portion, the media thins, and connective tissue replaces smooth-muscle cells. As the aneurysm is continually subjected to vascular blood pressure and blood flow, the aneurysm will grow outwardly from the wall of the vessel, which can cause pressure on the surrounding tissue as the sac or fusiform contacts the surrounding tissue. When the malformation occurs in the brain, this pressure can lead to serious mass effects, such as cognitive impairment, loss of vision, and nerve palsies. Additionally, as the aneurysm is subject to vascular blood pressure and blood flow, the walls of the aneurysm weaken, usually in the dome portion, which can eventually cause the aneurysm to tear or rupture. Ruptured aneurysms are the most common cause of subarachnoid hemorrhages, which have a mortality rate of approximately 50%.
Aneurysms and other malformations are especially difficult to treat when located near critical tissue or where ready access to the malformation is not available. Both difficulty factors apply especially to cranial aneurysms. Surgical methods have developed to treat cranial aneurysms and generally include eliminating blood flow to the aneurysm by placing a clip around the neck of a saccular aneurysm or by blocking off a fusiform aneurysm by cliping both ends of the fusiform and detouring blood flow around the secluded fusiform through an implanted vessel graft. Due to the sensitive brain tissue surrounding cranial blood vessels and the restricted access, it is challenging and risky to surgically treat defects of the cranial vasculature.
Alternatives to such surgical procedures include endovascular delivery of an implantable device, such as a stent-like device or embolic coil, through a microcatheter delivery device. In one such procedure to treat a saccular-form cranial aneurysm, the distal end of an embolic coil delivery catheter is initially inserted into non-cranial vasculature of a patient, typically a femoral artery in the groin, and guided to the aneurysm. The aneurysm sac is then filled with embolic material, such as platinum coils, that forms a solid, thrombotic mass that protects the vessel walls from blood pressure and flow. This treatment method is advantageous in that it only occludes blood flow to the aneurysm leaving the surrounding portions of the vessel unobstructed. However, it cannot treat fusiform aneurysms, and the aneurysm volume is permanently maintained.
Another technique involving the use of an intravascular implant delivers, by a microcatheter, an occlusive device in the form of a tubular, stent structure. Stents can be braided, woven, or wound from various filaments, such as a wire or wires, laser-cut from metal, or made in various other ways. They can either be self-expanding or can be expanded by another device such as a balloon. What most have in common is radial symmetry, i.e., a uniform porosity, meaning that they do not cover one portion, side, or radial sector of the vessel more or less porously than other sectors. Their symmetric construction, and therefore coverage of vessel walls, is relatively homogeneous around any given transverse slice or cross-section.
This homogenous structure can be disadvantageous in that such stents not only occlude or block blood flow to the aneurysm, but they also block blood pressure and flow along the entire length of the stent, which often impedes flow into surrounding joined vessels, such as perforator-type vessels branching off of the parent vessel. The use of a non-discriminatory occlusive device in this type of vessel can cause unintended harm to the patient if the openings, or ostia, of the perforator vessels are blocked.
Some have developed selectively-occlusive devices that discriminately block flow to an aneurysm while simultaneously allowing flow to surrounding vessels. These attempts to create discriminate occlusion devices have used multilayered devices, varied the amount of filaments along the length of the intravascular implant, or changed the picks per inch along the length of the intravascular implant. But, generally, these devices face difficulties in manufacturing and increased costs due to difficulties in creating the multiple layers or variations in the number of filaments to create the varied porosity regions.
Accordingly, there remains a need for a device that effectively occludes a neck or fusiform of an aneurysm or other arteriovenous malformation in a parent vessel without blocking flow into perforator vessels communicating with the parent vessel that is structurally sound and easily manufactured.